New results from an examination of 15 bright galaxies finds that they have too …

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A study that appeared in last Friday's edition of Astrophysical Journal Letters reveals a new clue about evolution of galaxies; unfortunately, that clue does not fall in line with our current theories about galactic evolution. It is currently thought that massive luminous galaxies—such as the Milky Way—began life shortly after the big bang. Accordingly, the stars in these galaxies have been turning simply hydrogen in heavier elements for many billions of years.

But, using results from the KPNO International Spectroscopic Survey (KISS) project, researchers have found a handful of galaxies that have too little metal (in astronomy terms, elements heavier than helium) for their given brightness. The implication of the low metal abundance is that the galaxies are young, which challenges the prevailing notion that such bright galaxies should be considerably older. The difference is rather dramatic, as the metal content suggests an age of only 3 or 4 Gyrs, instead of the 9 or 10 Gyrs that traditional models would suggest.

There aren't a lot of these anomalous readings—the authors found a total of about 15 in a survey of over 53,000 galaxies. Still, they do demand an explanation, and the authors suggest several.

The first is the most obvious: these are galactic late bloomers, massive objects that resisted gravitational collapse and stellar formation for far longer than their counterparts elsewhere in the universe. A second possibility is that these observations result from a sudden output of light from a stellar merger or extreme starburst in an otherwise normal dwarf galaxy.

The low metal content is in line with other dwarf galaxies, but the brightness is not. This scenario is unlikely, however, as the researchers calculate that it would require a luminosity enhancement factor of nearly 28, far off the high-end of what is seen in even the most powerful starbursts (these exhibit luminosity enhancements of two- to three-fold). Finally, they postulate that these anomalous bodies could be the result of a normal L* galaxy accreating a very large amount—an order of magnitude more than the galaxy weighs—of pristine gas.

While any of the scenarios is possible, the latter two seem pretty unlikely. Since the galaxies included in this study are nearby—in astronomical terms, at least—with redshifts between 0.29 and 0.42, they could provide a useful and unique insight into the formation of galaxies in the early stages of life, assuming that the late bloomer idea is the correct explanation.

According to the lead author of the paper, John Salzer, "these objects may represent a unique window on the process of galaxy formation, allowing us to study relatively nearby systems that are undergoing a phase in their evolution that is analogous to the types of events that, for most galaxies, typically occurred much earlier in the history of the Universe."

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Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm